A 20-year study of monkeys has found that cutting calories by almost a third slowed their aging and fended off death. The study suggests that a similar diet might work in humans.

Since the 1930s, researchers have conducted studies to try to understand an intriguing finding - why mice that eat a calorie-restricted, but healthy diet live longer than rodents that consume a normal diet.

Now, in the largest, most highly controlled study to date, researchers at the Wisconsin National Primate Research Center have confirmed that monkeys that eat a calorie-restricted, well-balanced diet live longer, healthier lives than animals on unrestricted diets.

It is the first time that the anti-aging effects of calorie restriction have been seen in a non-human primate - rhesus monkeys.

Researcher Richard Weindruch with the Geriatric Research Education Clinical Center at the Veterans Administration Hospital in Madison explains.

"Our data show that there's about a three-fold higher risk of developing a disease of aging in those animals fed the normal diet as opposed to those that have been on caloric restriction since they were adults," said Richard Weindruch.

Age-related diseases among monkeys that were allowed to eat anything they wanted included cancer, diabetes and cardiovascular disease, shortening their lifespans. The incidence of some of these diseases was cut almost in half for monkeys on a restricted diet.

The study, which began in 1989, followed an initial group of 30 adult primates and was expanded in 1994 to include another 46 mature rhesus macaques.

An article on the work is published in the journal Science.

Weindruch says the primate study will continue to see how long the monkeys live eating fewer calories.

So far, 37 percent of the monkeys that kept their regular diet have died of age-related diseases, compared to only 13 percent for monkeys that ate one-third fewer calories. A few monkeys died of unrelated conditions, such as injury.

Weindruch says researchers do not know why calorie restriction increases life span, but they think there is a beneficial shift in the way the body processes energy in the monkeys that eat fewer calories.

Weindruch says the research shows that not only is life extended in the calorie-restricted monkeys, but their quality of life also is improved, with a delay in muscle loss and brain shrinkage that can lead to dementia.

"The upshot is that now we know that the health benefits of caloric restriction and its ability to oppose the aging process extends into primates that now allow us to probe mechanisms of aging that have relevance to humans," he said.

The difference in the appearance of calorie-restricted monkeys and those on a normal diet is striking. Primates that ate fewer calories look younger and healthier than fatter, frumpier monkeys on an unrestricted diet.